Feed gases that result from the processing of crude carbonaceous or hydrocarbonaceous fossil fuels are often contaminated with certain undesirable components. These undesirable components are native to the crude carbonaceous or hydrocarbonaceous fossil fuels or originate as byproducts of the aforesaid processing. The production of a raw synthesis gas from the gasification of coal is an illustrative example. Coal is a carbonaceous fossil fuel comprising a complex, extended polymeric network of compounds containing significant molar quantities of carbon (C) and hydrogen (H), along with minor molar quantities of nitrogen (N), sulfur (S) and other elements. The production of synthesis gas, a mixture of carbon monoxide and hydrogen, involves, in one method, the partial oxidation of coal with air or pure oxygen. Under appropriate conditions of temperature and pressure, the coal is volatilized to a raw synthesis gas comprising proportionately large volumes of carbon monoxide (CO) and hydrogen (H2) gases, as well as smaller volumes of minor gaseous components, such as, for example, hydrogen sulfide (H2S), carbon oxysulfide (COS), hydrogen cyanide (HCN), oxides of nitrogen (NOx) and carbon dioxide (CO2). Most of these minor gaseous components are the result of the reaction conditions used in the partial oxidation of coal for the manufacture of synthesis gas acting upon the relatively small molar amounts of nitrogen and sulfur in the carbonaceous fuel. Collectively, these minor gaseous components may be referred to as acid gases owing to their acidic nature. As defined herein, an acid gas is a gas that can form a salt upon exposure to a solution containing a base.
These so-called acid gases, when present in an acid feed gas, present a potential problem in the downstream catalytic processing of the acid feed gas. For example, synthesis gas is a useful feedstock for the production of certain commodities such as ammonia, hydrogen, hydrocarbons, methanol, and aldehydes as well as other commodities. The processes used to manufacture such commodities from synthesis gas are often carried out by subjecting a stream of synthesis gas to specific conditions of temperature and pressure in the presence of a catalyst specific for the production of a given commodity, wherein a catalyst is defined herein as a species present in a chemical reaction that accelerates the rate of a chemical reaction, e.g. the conversion of a reactant to a product, without participating in the chemical reaction. The catalyst is thereby normally only required to be present in a fractional molar quantity relative to the reactant. The catalysts employed for these processes often comprise transition metals. Unfortunately, many of the transition metals are susceptible to chemically bonding strongly with certain species that are identified as catalyst poisons, e.g. sulfur- and cyanide-containing species. The formation of these strong chemical bonds results in an irreversible deactivation of the active sites on the catalytic metal, thereby eliminating the participation of the active sites in the catalytic chemical reaction; a phenomenon commonly known as catalyst poisoning. Catalysts are present, by definition, in proportionately small relative molar amounts in a reaction mixture and are therefore susceptible to catalyst poisoning by proportionately small relative molar amounts of a catalyst poison.
An additional concern with respect to acid gases stems from the effect of their requisite acidity upon the reactivity and selectivity of the transition metal catalysts employed in downstream catalytic processing. In particular, it will be noted that the precise nature of the products produced by transition metal catalysts and the efficiency by which the transition metal catalysts carry out their transformation is markedly dependent upon the relative acidity of the immediate environment surrounding the active catalytic site. For the reasons cited above, it is desirable to remove these acid gases in as complete a manner as possible prior to any downstream processing.
Conversely, removal of some base gas component from a base feed gas stream, comprising at least one base gas and at least one nonbase gas, is desirable for reasons similar as those adduced above. For example, in the well-known hydroisomerization of certain petroleum distillates, acidic sites are required to donate free protons to effect the isomerization; acidity is therefore key to the process and removal of base gas contaminants is a vital part of preserving the integrity of the employed catalyst.
Many conventional processes extant in the art of acid gas removal utilize gas scrubbers, whereby a gas comprising an acid gas component is contacted with a basic solution and washed free of the acid gas component in the process. Pressure swing absorption is another common method used in the art, whereby a gas stream is passed over a bed of sieves in which larger gas molecules can become trapped, while allowing the smaller species, such as hydrogen, to pass through. Cryogenic distillation has also been used in the art, whereby a gas stream is cooled and liquefied in the process, the cooled mixture is then distilled to separate and remove select components from the gas stream. Previous art highlighting these and other processes will be outlined below.
In U.S. Pat. No. 5,319,924, chlorine-containing gases are removed from a synthesis gas stream by contacting the synthesis gas stream with scrubbing water comprising a solution of a base selected from the group comprising ammonia (NH3), ammonium hydroxide (NH4OH), sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium carbonate (Na2CO3) and potassium carbonate (K2CO3). Additionally, substantially all of the sulfur-containing gases e.g. hydrogen sulfide (H2S) and carbon oxysulfide (COS) are removed in a conventional acid gas removal zone comprising suitable conventional processes involving refrigeration and physical absorption with solvents, such as methanol, N-methylpyrrolidone, triethanolamine, propylene carbonate, or chemical absorption with solvents such as amines or hot potassium carbonate.
Other solvents for chemical absorption of acid gases, wherein a chemical reaction, most often salt formation, takes place between the absorption solvent and the acid gas, may also be used. For example, U.S. Pat. No. 6,207,121 B1 discloses a composition and process for removal of acid gases, wherein alkanolamines of the formula RNHCH2CH(OH)CH2CH3 or mixtures thereof in combination with a tertiary alkanolamine are effective in the removal of acidic gases from a fluid stream containing same. In one aspect, the invention is an aqueous solution adapted for use in the removal of acidic gases from a fluid stream containing same, said aqueous solution comprising an effective amount of an alkanolamine having the formula shown above. In another aspect of the invention is a process for removing acid gases from a fluid stream containing same, said process comprising contacting said fluid stream containing acidic gases with an aqueous solution comprising an effective amount of an alkanolamine of the formula shown above. The alkanolamines are found to be effective for removing acidic gases, particularly CO2, H2S, COS or mixtures thereof, from a fluid stream containing same. The process of the present invention can be carried out in any conventional equipment suited for the removal of acidic gases from fluids.
U.S. Pat. No. 4,284,423 discloses a process for the separation of carbon dioxide and other acid gas components in a compound distillation column, from a gaseous mixture comprising hydrocarbon feeds containing admixtures of methane and hydrogen. It is the primary objective of this invention to provide a new and improved distillation process for the separation in a compound column of acid gas components from hydrocarbon streams. A particular object of the '423 patent is to provide a process wherein carbon dioxide can be separated from methane gas streams by distillation in a compound column, or column wherein the distillation is carried out in two or more sections (or zones) operated at different pressures. A more specific object of the '423 patent is to provide a process of such character for the more effective separation of carbon dioxide from gaseous methane streams, notably gaseous streams wherein methane is contained or provided in admixture with carbon monoxide and hydrogen. In the operation of the separation process of the '423 patent, the carbon dioxide containing feed gas is preferably introduced into the low pressure section of the column and the total pressure in this section is maintained below the critical pressure of carbon dioxide, the primary component of the liquid bottoms. In accordance with such a process it becomes feasible to effect almost complete separation of carbon dioxide and other acid gas components from a methane-containing feed gas such as natural gas or synthesis gas.
The removal of HCN by use of an absorption column and, optionally, the removal of H2S, COS and CO2 by conventional processes, including refrigeration and/or physical or chemical absorption, are also known in the art. U.S. Pat. No. 4,189,307 discloses a process for the production of a clean HCN-free synthesis gas from hydrocarbonaceous fuel. One embodiment of the process comprises contacting the clean gas stream with an aqueous absorbent in liquid phase in an HCN-absorption zone using a conventional gas-liquid contacting apparatus as the HCN-absorber. Fresh HCN-free aqueous absorbent is, optionally, introduced into the top of the column in order to give a final clean rinse to the gas stream that leaves the column containing substantially no HCN. Aqueous absorbents that may be used include dilute aqueous solutions of sulfuric acid, alkali carbonates, alkali bicarbonates, or mixtures thereof. In another embodiment of the process of the '307 patent, optional steps are provided for removing acid gases such as H2S, COS and CO2, if present in the HCN-free synthesis gas, by introduction into an acid-gas purification zone. Any suitable conventional purification process may be used to remove at least one acid gas from the HCN-free synthesis gas including, for example, refrigeration and/or physical or chemical absorption with a liquid organic solvent. Typical liquid solvent absorbents include: methanol, N-methylpyrrolidone, triethanolamine, propylene carbonate, or hot potassium carbonate.
U.S. Pat. No. 4,536,382 discloses a process that provides for; (a) the conversion of H2S from a synthesis gas stream; (b) the removal of CO2 from a water gas shifted synthesis gas stream and; (c) optionally, provides for the removal of minor quantities of COS from gaseous streams comprising hydrogen sulfide, hydrogen, carbon monoxide and carbon oxysulfide (COS). In particular, the process that provides for the conversion of H2S from a synthesis gas stream comprises contacting a gaseous stream containing H2, CO and H2S with an H2S-selective absorbent in an absorption zone and absorbing the bulk of the H2S in said stream, thereby producing a partially purified gas stream containing a minor portion of H2S. Any of the known H2S-selective absorbents may be employed including aqueous solutions of alkali metal carbonates, phosphates, diethylene glycol monoethyl ether and certain alkanolamines. In an additional embodiment, the partially purified gas stream containing a minor portion of H2S is subjected to conditions sufficient to provide for the water gas shift conversion of CO and water to H2 and CO2, thereby producing a modified gas stream having an increased ratio of H2 to CO and a minor quantity of H2S; said modified gas being treated under appropriate conditions with an absorbent selective for CO2 in the presence of H2 and CO, said absorbents including alkanolamines, sodium or potassium carbonate solutions, potassium phosphate, or solutions of sterically-hindered amines in aqueous or organic solvents, or in combinations of amines and potassium carbonate to produce CO2-rich absorbent that is regenerated, the CO2 being thus recovered. In yet another embodiment, the remainder of the H2S in the modified gas stream is removed by contacting the stream with a specific oxidizing reactant selected from the group comprising the oxidizing polyvalent metal chelates or chelates of nitrilotriacetic acid, in particular, the chelates employing iron, copper and manganese are preferred but those employing lead, mercury, palladium, platinum, tungsten, nickel, chromium, cobalt, vanadium, titanium, tantalum, zirconium, molybdenum, and tin are also acceptable. Solutions of these metal chelates are responsible for converting H2S in the modified gas stream in the contacting zone to sulfur, and recovering a substantially sulfur-free gas stream having an increased ratio of H2 to CO. Additionally, an optional embodiment provides for the removal of minor quantities of COS from the streams in a COS conversion zone. The hydrolysis of COS to H2S and CO2 is shown by the following formula:COS+H2O→H2S+CO2.
Water is added, in the COS conversion zone, and reacts with the COS in the presence of a preferred catalyst, such as those including the metals Ni, Pd, Pt, Co, Rh or In. Platinum on alumina is a preferred catalyst and support system. The H2S produced by hydrolysis is removed by contacting the stream with a specific oxidizing reactant as outlined hereinabove.
International application WO 99/14473 discloses a high temperature desulfurization embodiment as part of an integrated gasification combined cycle (IGCC) wherein the combustion gas that enters the high temperature desulfurization system has already had the majority of its sulfur content removed through one of the conventional low temperature desulfurization processes well known to those skilled in the art. Many of these processes include a COS hydrolysis step to increase the sulfur removal by about 1–2%. Utilizing the high temperature desulfurization system described herein eliminates the need for COS hydrolysis and removes the last traces of sulfur.
U.S. Pat. No. 6,090,356 teaches an integrated process that removes acidic gases such as H2S, COS and CO2 from raw synthesis gas. The process comprises separating H2S and COS from a raw synthesis gas by absorption with a liquid solvent, removing coabsorbed CO2 by stripping the solvent with nitrogen, separating the H2S and COS from the solvent and recovering sulfur from the H2S and COS. This separation is accomplished by sending the synthesis gas to an acid gas recovery unit where it is first treated in a H2S absorber that uses a liquid solvent for the removal of H2S. Significant amounts of CO2 are also removed by the H2S solvent in the first H2S absorber. The invention also comprises operative steps for removing CO2 from a shifted synthesis gas comprising CO2 and H2. To recover the CO2 absorbed in the acid gas solvent, also referred to as the “rich solvent,” the rich solvent is heated and the pressure reduced to desorb the CO2.
A process for using a catalytic hydrolysis of HCN to ammonia and carbon monoxide is known for removal of HCN in feed gas. In particular, U.S. Pat. No. 5,968,465 discloses a process for removal of HCN from synthesis gas by contact with a metal oxide catalyst comprised of the oxides of molybdenum, titanium, and aluminum in the presence of water vapor, and subsequently water washing the resulting gas. Reaction conditions for HCN removal include elevated temperatures and elevated pressures, and at these conditions at least about 95% of the HCN contained in the synthesis gas feed stream is removed. Subsequent to the contacting step, the gas is scrubbed with water to remove the formed NH3; the hydrolysis being necessary as ammonia is readily soluble in water while hydrogen cyanide is difficult to remove from synthesis gas because of its low solubility in common solvents, e.g., water.
U.S. Pat. No. 5,980,858 discloses a method and apparatus for treating wastes to produce synthesis gas, wherein, in one aspect of the invention, said synthesis gas is scrubbed for removal of acid gas components in a water scrubber before a water gas shift reaction. In another aspect of the invention, the process of removal of acid gas components from a water gas shifted synthesis gas produced from the wastes takes place in an acid gas remover, wherein the process of removal of acid gas components comprises a physical absorption process that is carried out to remove impurities including hydrogen sulfide (H2S), carbon oxysulfide (COS), and carbon dioxide (CO2).
As the current invention involves a gas permeable membrane, it will be useful to discuss the theory of membrane separations; specifically, separations of gaseous mixtures using membranes. Membranes are thin film barriers; the defining property of which is their ability to selectively allow certain components of a mixture to pass through the membrane while excluding from passage certain other components of a mixture based on differing rates of mass transport through the membrane for varying components. The exact mechanism of mass transport through a membrane is dependent upon the characteristics of that membrane; in particular, the chemical composition, the morphology of the material, and if applicable, the porosity. Nonetheless, all separations using membranes are governed by, to at least some extent, Fickian diffusion, wherein the driving force for transport of a substance is a gradient in chemical potential. The pressure differential across the membrane represents the gradient in chemical potential and therefore the driving force for diffusion through the membrane. To sum, the flux for a given component across a membrane is proportional to the pressure differential and inversely proportional to membrane thickness. Thus, most membranes are made to be very thin to attain high rates of separation.
For a mixture of two gaseous components in a feed, separation can only be effected when the partial pressure of a component in the feed exceeds the partial pressure of that same component in the permeate. Thus, membrane-mediated gas separations are also pressure-driven processes. This is typically accomplished in one of two basic ways: a high partial pressure of a component on the feed side can be achieved by realizing a high total pressure on the feed side; conversely, a low partial pressure of a component on the permeate side can be achieved by realizing a low total pressure on the permeate side. Because of the need for the continuous maintenance of a pressure differential to drive the separation in membrane mediated processes, practical membrane separations are continuous flow processes that feature the constant addition of feed at high total pressure or the constant removal of permeate at low partial pressure for the permeating component of interest.
Typical membranes are specially prepared and designed with their ultimate end use in mind. Careful control over such factors as porosity, polymer molecular weight, tensile strength and the like have led to a limited number of specialty gas permeable membranes that are offered commercially. The extensive optimization required to properly fashion a membrane of good efficiency in a separation of a specified solute has necessarily limited the number of membranes available commercially. Membranes do, however, have significant advantages over conventional separation technologies. A lack of moving parts makes them mechanically simple and energy efficient. Membranes typically arrive in modular packages and therefore require little space as compared to their conventional counterparts. A great deal of effort has been devoted to the investigation and discovery of selective membranes; indeed, nearly all useful membranes are selective for the preferential permeation of a specific component. In the case of gas permeable membranes, there exist membranes of highly specific design for the selective permeation of say, hydrogen over methane.
The use of gas permeable membranes for the separation of hydrogen, carbon monoxide and the acid gas carbon dioxide is documented in the art. For example, U.S. Pat. No. 5,322,617 discloses an improvement for the treatment of heavy oil/water emulsions with carbon monoxide under water gas shift reaction conditions, and recovering not only the upgraded heavy oils but also hydrogen and carbon dioxide. According to the '617 patent, the excess carbon monoxide may be recovered, e.g. the carbon monoxide produced may be removed by a membrane separation process. The hydrogen and carbon dioxide produced may also be recovered by a membrane separation process.
Removal of gases, including the acid gas CO2, using a selective membrane has been disclosed. U.S. Pat. No. 5,647,227 to Lokhandwala teaches a membrane separation process combined with a cryogenic separation process for treating a gas stream containing methane, nitrogen and at least one other component. The membrane separation process of the '227 patent works by preferentially permeating methane and the other component and rejecting nitrogen. In one aspect, the invention of the '227 patent is a process for treating a gas stream containing methane, nitrogen and at least one other component. The process comprises a membrane separation step followed by a cryogenic separation step. The membrane step works by preferentially permeating methane and one or multiple components that might affect the cryogenic separation, and by rejecting the nitrogen component of the stream. The driving force for transmembrane permeation is provided for by a superatmospheric pressure on the feed side, a subatmospheric pressure on the permeate side, or the combination of both. The effects of the membrane separation step are two-fold: (i) to remove contaminants that might interfere with the operation of the cryogenic nitrogen/methane separation unit, such as by freezing out during refrigeration prior to entry into the distillation column or within the distillation column itself, and (ii) to reduce the volume of gas to be treated by cryogenic separation.
A process and article of manufacture for micro-distillation of acid anions including cyanide, arsenate and sulfide, by acidification of the anion in the lower elongated member of the article of manufacture and volatilization to the acid gas followed by permeation through a gas permeable membrane and collection in a caustic solution held in the upper elongated member of the article of manufacture is disclosed in U.S. Pat. No. 5,160,413. In particular, with respect to the process, a sample of the cyanide along with water and an acidifying agent is mixed together and placed in the lower elongated member. The assembled distillation apparatus is then placed in a heating member where sufficient temperature is provided to vaporize hydrogen cyanide gas from the sample mixture. The hydrogen cyanide gas passes upwardly from the lower elongated member through the permeable membrane into the upper elongated member. The upper elongated member contains a salt forming material whereby a cyanide salt is formed.
Separation and/or removal of carbon monoxide, carbon dioxide and hydrogen gases from a water shifted synthesis gas by a scrubbing process, a pressure swing absorption process or by a membrane separation process has been disclosed in U.S. Pat. No. 5,322,617. The invention is an improvement upon the treatment of heavy oil/water emulsions with carbon monoxide under water gas shift reaction conditions to recover the upgraded heavy oils, as well as hydrogen and carbon dioxide. In one feature of the invention, the carbon monoxide may be recovered, e.g. the carbon monoxide produced may be removed either by a scrubbing process, or by a pressure swing absorption process, or by a membrane separation process. The hydrogen and carbon dioxide may also be removed either by a scrubbing process, or by a pressure swing absorption process, or by a membrane separation process.
Thus, the successful removal of acid or base gases from acid or base feed gases remains a problem heretofore not completely solved by the prior art. The present invention is intended to address the deficiencies and shortcomings cited hereinabove by providing a novel process by which acid or base gases may be removed from an acid feed gas or a base feed gas, respectively.